There are several reasons why this can happen, the most common is your capture resolution. At the time of this writing the windows capture methods struggle to capture high resolutions under certain circumstances, as a general rule of thumb anything greater then 1200p (1920x1200) starts to suffer from performance issues.

Another cause can be how the game or application you are running is configured. Because of the way windows integrates with the WDM (Windows Desktop Manager) running applications in "Full Screen" mode may in some cases cause a large performance penalty. Try switching to windowed full screen mode, the difference in performance can be like night and day. Titles known to be affected by this are: Fallout76, The BattleField Series and SCUM.

Some titles do some strange things at early initialisation that cause capture performance issues. One such title is the Unigine Valley benchmark where the capture rate is limited to 1/2 the actual rate. For an unknown reason to both myself and the Unigine developers a simple task switch (alt+tab) in and out resolves the issue. This is not a Looking Glass bug.